Carbon dioxide (CO2) is used in a wide variety of industrial processes and products from the carbonation of beverages to the generation of semiconductors. Industrial applications for CO2 include wafer cleaning, removal of residual photoresist, low K dielectric annealing and cleaning, use as laser gases, particle removal and plasma generation. The presence of contaminants in such ultrahigh purity applications can result in products that are unusable in the case of semiconductor applications or the damage of optics and lasers in other relevant applications. Various phases of CO2, liquid, gas and supercritical, are used dependent upon the application.
Purification of CO2 to a high level is difficult as CO2 is inherently wet. Gases can be readily acquired that are ≧99.999% pure (contaminants ≦10 parts per million, ppm; Ultra High Purity); however, there is a need to further purify gases to ≦5 parts per billion (ppb) contaminants to meet the current Semiconductor Industry Association guidelines. Future requirements will be for contaminants of ≦1 ppb, preferably ≦0.1 ppb. Contaminants of concern to the semiconductor industry include moisture, hydrocarbons, particulates and metals. Other contaminants include oxygen-, nitrogen-, sulphur- and phosphorus-containing compounds such as O2, NOx, SOx, COS, and POx, wherein x≦3, and corresponding organoheteroatom derivatives wherein heteroatoms include, but are not limited to, oxygen, nitrogen, sulphur, phosphate and silicon.
Purification of CO2 to a high level is even more essential when it is used in the supercritical state. A supercritical fluid is a fluid which is in a state above its critical temperature and critical pressure where the gas and liquid phases resolve into a single medium, in which density can vary widely without a phase transition. This allows, for instance, for substances that normally act as solvents primarily for inorganic or polar substances to also become efficient solvents for organic or non-polar materials. The supercritical state of CO2 can be reached under relatively moderate conditions at a critical point of 31.3° C., (88.3° F.) and 74 barr (1070 psi).
Supercritical CO2 is useful as a cleaning agent because it is able to enter small features on surfaces and porous interior surfaces to remove contaminants, etched photoresist and other undesirable materials from substrates. However, this property can also result in impregnation of wafers and other high purity substrates with contaminants present in the CO2 stream. Additionally, the high pressure and temperature of the fluid can result in some contaminants becoming more nefarious. For example, water and oxygen in the context of supercritical CO2 can become highly corrosive, whereby desired structural features on wafers are subject to degradation.
Equipment used in the semiconductor industry can also act as a source of contaminants. Stainless steel components can leach metals including iron, chromium and nickel as metal complexes or metal ions. Leached metals are volatile at low concentrations, ppb to parts per trillion (ppt), and are readily captured in the gas phase resulting in potential contamination of silicon wafers or other high purity products. Therefore, all equipment for use in the semiconductor manufacturing process must be thoroughly cleaned to remove potential surface contaminants.
Spiegelman et al (U.S. Pat. No. 6,361,696, incorporated herein by reference) teach the use of high silica zeolites for the continuous purification of CO2 in a dual bed apparatus. Although the high silica zeolites are able to remove heavy hydrocarbons from CO2 efficiently, removal of other contaminants is limited. Lansbarkis et al. (U.S. Pat. No. 6,511,528) teach the use of a series of materials to remove a series of contaminants from CO2 to all for its use in the semiconductor industry. The materials may be placed in a single or multiple containers. Either arrangement results in a complex system. If the materials are placed in a single container, the container must be discarded upon the breakdown of the least stable purification material. If multiple containers are used, complex replacement schedules must be followed to ensure the overall purity of the CO2.